1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive device including an electrode layer adhering to the upper surface of a substrate and a piezoelectric/electrostrictive layer adhering to the upper surface of the electrode layer, and to a method for manufacturing the same.
2. Description of the Related Art
A piezoelectric/electrostrictive device having such a structure has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-284677. The piezoelectric/electrostrictive device has a metal-based electrode layer in tight contact with the surface of a ceramic substrate mainly containing zirconia, and piezoelectric/electrostrictive layer in tight contact with the electrode layer. The piezoelectric/electrostrictive device having such a structure is generally manufactured by the following process.
First, an electrode layer or an electrode precursor coating (turned to an electrode layer by heat treatment) is applied onto the surface of a substrate (first layer forming step, the electrode layer or the electrode precursor coating hereinafter referred to as the first layer). Then, a second layer containing a piezoelectric/electrostrictive material is formed on the first layer (second layer forming step). Subsequently, the multilayer composite prepared by the steps up to the second layer forming step is heated to a firing temperature to fire at least the second layer (firing step). If the first layer is the electrode layer, the firing step fires only the second layer; if the first layer is the electrode precursor coating, the firing step fires both the first layer and the second layer. After the firing step, the multilayer composite is cooled at a temperature decreasing rate of at least the rate of natural cooling (cooling step).
In order for the piezoelectric/electrostrictive device to exhibit a variety of advantageous characteristics, including not only piezoelectric/electrostrictive characteristics, but also environmental characteristics, such as possibility of using no harmful chemicals, various materials can be used for the piezoelectric/electrostrictive layer and the substrate.
However, the thermal expansion coefficient of the piezoelectric/electrostrictive layer may be larger than that of the substrate, depending on the combination of their materials. In such a condition, the piezoelectric/electrostrictive layer tends to shrink more than the substrate in the cooling step, while the substrate suppresses the shrinkage of the piezoelectric/electrostrictive layer because of the small shrinkage of the substrate. Consequently, tensile stress is produced in the piezoelectric/electrostrictive layer.
A large difference between the thermal expansion coefficient of the piezoelectric/electrostrictive layer and the substrate produces a large residual tensile stress in the piezoelectric/electrostrictive layer in the known process, and accordingly the piezoelectric/electrostrictive characteristics are degraded. In addition, the piezoelectric/electrostrictive layer may be cracked during the cooling step or a subsequent polarization step.